Modeling heterojunction bipolar devices

A new analytical compact circuit model for double heterojunction bipolar transistors (DHBTs) is developed based on extensive theoretical analysis and some experimental study of the band structure of Si/SiGe and InP based DHBTs. This model can replace the conventional SPICE Gummel Poon model for low-risk analog and mix-signal design with bipolar devices.; Because of the double hetero-structure, the conduction band of an npn transistor will have two spikes in both base-emitter and base-collector junctions. The first heterojunction (base-emitter junction) is advantageous for the suppression of the hole injection and thus provides high current gain. However, the inherit barrier of the second heterojunction (the base-collector junction) blocks the electron transport and will cause earlier current roll-off in the quasi-saturation region of the current-voltage characteristics. The second barrier changes the boundary condition of the Gummel Poon model. It also changes the definition of the knee current of the DHBTs resulting in that the knee current in DHBT will not be a reflection of high-level injection, but a effect of the second barrier effect.; The analytical current-voltage equations are derived by the physical principle of the DHBT structures. We introduced two parameters B and b to characterize the heterojunction. Parameter B is a reflection of the barrier height and it can be fitted by experimental measurement, then the barrier height will be determined. The other b relates to the collector side base doping of non-uniform doped base, which can be determined by experiment as well. We also introduced two more parameters AF,AR which modified both forward and reverse saturation currents. The two most important ideality factors nF,nR are redefined.; In addition to the derivation of this model, we have also carried out experimental measurements and developed a practical parameter extraction procedure to get the necessary parameter values. This model was implemented to circuit design tools HSPICE and HP-ADS and applied to SiGe DHBTs and InP-based DHBT measurements. The simulation results from this model for both of these two kinds of material system fit the experimental data very well.